Research

Current Projects / Reliability Testing of Electronic Materials

 

Various mechanical properties of electronic materials including - elastic constants, - residual stress, - fatigue properties, - fracture toughness, - interface strength for multilayers can be measured with bulge testing. A schematic of the bulge apparatus is given in Figure 1, where pressure is applied to a free-standing diaphragm, and resulting deflections in the diaphragm are measured with a laser sensor.

Figure 1. Schematic of the bulge test apparatus. Click image to enlarge

Specimen preparation is summarized in Figure 2. The free-standing diaphragm is obtained by coating a Si chip with any desired material and etching a hole, usually circular, in the Si substrate over which the coating is suspended. The sample is then attached on top of the hydraulic oil chamber shown in Figure 1 and pressure is applied by displacing hydraulic oil with a piezoelectric actuator. Pressure can be increased monotonically, or depending on the frequency of the driving voltage, a cyclic pressure can be applied for fatigue tests. Loading curves in Figures 3 and 4 show the resulting pressure vs. deflection data of the aromatic polyimide PMDA-ODA for monotonic and fatigue loading.

Figure 2. Specimen preparation.(polyimide diaphragm and Al coating) Click image to enlarge

Figure 3. Pressure vs. mid-point deflection curve for the aromatic polyimide PMDA-ODA. Elastic modulus and residual stress are extracted from the readings. The inset shows specimen geometry. Click image to enlarge

Figure 4. Pressure and mid-point deflection data for fatigue loading plotted as a function of time. Specimen is similar to that given in Figure 2. Pressure is cycled between 0.029-0.182 MPa with a saw-tooth-shaped time dependency Click image to enlarge

We also developed a technique to keep a constant strain rate during loading of the diaphragm samples. Constancy of strain rate is especially important for the measurement of strength. In thin film testing, strain control is a challenging task. But it proves to be worth the extra effort. For a strain rate window of we found a remarkable dependence of strength on strain rate for thin Au films at room temperature.

Figure 5. The evolution of strain as a function of time in a series of controlled experiments. Click image to enlarge

Figure 6. Strain-rate dependence of yield strength in 500-nm thick Au films. Click image to enlarge